1. Field of the Invention
The present invention relates to calibration technologies and it particularly relates to a calibration method for correcting the mismatch in a plurality of antennas and also particularly relates to a base station apparatus, a terminal apparatus and a radio apparatus utilizing the same.
2. Description of the Related Art
An OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is one of multicarrier communication schemes that can realize the high-speed data transmission and are robust in the multipath environment. This OFDM modulation scheme has been used in the wireless LAN (Local Area Network) standards such as IEEE802.11a/g and HIPERLAN/2. The burst signals in such a wireless LAN are generally subject to the effect of frequency selective fading. Hence, a receiving apparatus generally carries out the channel estimation dynamically.
In order for the receiving apparatus to carry out the channel estimation, two kinds of known signals are provided within a burst signal. One is the known signal, provided for all carriers in the beginning of the burst signal, which is the so-called preamble or training signal. The other one is the known signal, provided for part of carriers in the data area of the burst signal, which is the so-called pilot signal (See Reference (1) in the following Related Art List, for instance).
Related Art List
    (1) Sinem Coleri, Mustafa Ergen, Anuj Puri and Ahmad Bahai, “Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems”, IEEE Transactions on broadcasting, vol. 48, No. 3, pp. 223-229, September 2002.
In wireless communications, adaptive array antenna technology is one of the technologies to realize the effective utilization of frequency resources. In adaptive array antenna technology, the amplitude and phase of signals transmitted from and received by a plurality of antennas, respectively, are so controlled as to form a directional pattern of the antenna. Hereinafter, such a directivity pattern will be referred to as “adaptive pattern”. One of techniques to realize higher data transmission rates by using such an adaptive array antenna technology is the MIMO (Multiple-Input Multiple-Output) system. In this MIMO system, a transmitting apparatus and a receiving apparatus are each equipped with a plurality of antennas, and a channel corresponding to each of the plurality of antennas is set. That is, channels up to the maximum number of antennas are set for the communications between the transmitting apparatus and the receiving apparatus so as to improve the data transmission rates. Moreover, combining this MIMO system with a technique such as the OFDM modulation scheme results in a higher data transmission rate.
The combination of antenna directivity patterns in a transmitting apparatus and a receiving apparatus in the MIMO system may be expressed as follows, for example. One case is that the antenna of the transmitting apparatus has an omni pattern whereas the antenna of the receiving apparatus has an adaptive pattern. Another case is that both the antenna of the transmitting apparatus and the antenna of the receiving apparatus have adaptive patterns. The system can be more simplified if the former is used whereas the antenna directivity pattern is controlled in a more detailed manner in the latter and therefore the characteristics thereof can be improved. Since in the latter case the transmitting apparatus carries out adaptive array signal processing for transmission, known signals for use with channel estimation are provided beforehand from the receiving apparatus. To improve the accuracy in the control of adaptive array antennas, the transmitting apparatus shall acquire the channel characteristics corresponding to all the combinations between a plurality of antennas included in the transmitting apparatus and a plurality of antennas included in the receiving apparatus. Accordingly, the receiving apparatus transmits from all of its antennas the known signals for use with channel estimation. Hereinbelow, the known signals for channel estimation transmitted from a plurality of antennas will be referred to as “training signals”, regardless of the number of antennas to be used for data communications.
Under these circumstances, the inventors of the present invention have come to recognize a problem as described below. If a radio apparatus including a transmitting apparatus therein performs adaptive array signal processing and consequently the mismatch between an analog circuit for transmission and an analog circuit for receiving (hereinafter referred to as “mismatch”) differs for each antenna, the directivity pattern at the time of transmission will differ from a desired pattern. That is, although the radio apparatus calculates, based on the received signals, the weights to form the transmission directivity pattern, a directivity pattern which is not the directivity pattern to be realized by the weights is realized due to the mismatch that differs per antenna. Thus, the calibration for a plurality of antennas needs to be conducted. Since the error contained in a calibration result leads to the deterioration of communication quality, the error contained in the calibration result needs to be reduced. Also, it is desired to execute a calibration when the radio apparatus is activated or the frequency to be used for communications is changed.